1. Field of the Invention
The present invention relates to a video signal processing circuit that can correspond to a plurality of television signal methods, which can generate a video signal where a signal format of television signal methods based on a SECAM Standard, a PAL Standard and/or an NTSC Standard is set.
2. Description of the Related Art
The SECAM Standard, the PAL Standard and the NTSC Standard are used as an analog television signal method. A specification corresponding to the SECAM Standard is set for a television receiver installed in a broadcast area of the SECAM Standard. A specification corresponding to the PAL Standard is set for a television receiver installed in a broadcast area of the PAL Standard. And, a specification corresponding to the NTSC Standard is set for a television receiver installed in a broadcast area of the NTSC Standard.
In recent years, a television broadcast based on a digital television signal method using a so-called MPEG method different from the analog television signal method has been actually used. The analog type television receiver corresponding to only the analog television signal method can not execute a reproduction based on the digital television signal method.
In the case of the execution of the reproduction based on the digital television signal, the digital type television receiver is used which corresponds to the digital television signal method. Also, since a converter (set top box) for converting the digital television signal into the analog television signal is installed on an input side of the analog type television receiver, the reproduction can be executed on the basis of the digital television signal through the analog type television receiver.
As the converter for converting the digital television signal into the analog television signal, the dedicated converter is prepared on the basis of the television signal method installed for the analog type television receiver. A converter for converting the digital television signal into the SECAM Standard of the analog television signal is used for the SECAM Standard of the analog type television receiver. A converter for converting the digital television signal into the PAL Standard of the analog television signal is used for the PAL Standard of the analog type television receiver. And, a converter for converting the digital television signal into the NTSC Standard of the analog television signal is used for the NTSC Standard of the analog type television receiver.
On the other hand, a color difference signal, on which a frequency modulation based on a frequency correlated for each level of the color difference signal that is one of base band signals is performed, is transmitted in the SECAM Standard. The level of the color difference signal and the frequency to be modulated are proportional to each other, as defined in accordance with a signal standard of the SECAM Standard. That is, as the level of the color difference signal is shifted from a low level to a high level, the frequency when the frequency modulation is done is proportionally increased. As for the relation between the level of the color difference signal and a color saturation, a color of a low color saturation and an achromatic color are assigned to a portion close to a center of the level. As the level of the color difference signal is increased or decreased from the center, a color of a high color saturation is assigned. In short, the SECAM Standard always includes a carrier signal containing the color of the low color saturation and the achromatic color (white and black portions). In the SECAM Standard, this inclusion brings about the occurrence of a dot interference on a receiving side, namely, a cross talk phenomenon of a carrier signal to a brightness signal. This occurrence causes the picture to be disgraceful as compared with the PAL Standard and the NTSC Standard. In the SECAM Standard, in order to avoid this disgracefulness, a transmitting side drops a signal amplitude in the vicinity of the achromatic color, for the color difference signal after the frequency modulation. Actually, a so-called bell filter is used as described below. That is, it outputs a signal, in which an amplitude of the color difference signal after the execution of the frequency modulation in the vicinity of the achromatic color is attenuated, with respect to the color difference signal (having a constant amplitude) after the execution of the frequency modulation in the vicinity of the achromatic color, when the color difference signal (having the constant amplitude) where the frequency modulation is performed on the basis of the level of the color difference signal is inputted. This signal process using the bell filter reduces the amplitude (strength) of the color difference signal after the execution of the frequency modulation in the vicinity of the achromatic color having the outstanding dot interference. Thus, this can reduce the influence on the brightness signal caused by the color difference signal in the vicinity of the achromatic color and thereby suppress the dot interference.
FIG. 8 shows a characteristic graph of the bell filter. A horizontal axis of FIG. 8 indicates a frequency, and a vertical axis thereof indicates a gain. A curve indicated by this characteristic graph is generated on the basis of the following equations:G=M0(1+j16F)/(1+j1.26F)F=f/f0−f0/f(f0=4.286 MHz)Here, f is a frequency, and M0 is a level of 23% of a peak white. In FIG. 8, a range from 3.906 MHz to 4.697 MHz is assigned as the frequency. However, this increase In the frequency is established such that it has a relation proportional to the level of the color difference signal that is one of the base band signals. In FIG. 8, a gain G is minimum at f=4.286 MHz. It is established that the gain G is attenuated with this frequency, namely, the level of the color difference signal corresponding to the achromatic color as a center. In this way, the bell filter corrects the color difference signal (having the constant amplitude) after the execution of the frequency modulation, to an amplitude determined for each frequency.
The bell filter can be constituted by a digital filter (FIR filter) having logical elements. FIG. 9 is a conceptual view around a conventional bell filter. An input signal Input shown in FIG. 9 represents a base band signal indicated by a first function F1. This base band signal and a frequency modulation carrier indicated by a second function F2 are synthesized to accordingly generate a frequency modulation signal Indicated by a third function F3. As the frequency modulation signal, a bell filter having a bell property indicated by a fourth function F4 generates a frequency modulation signal having a bell property indicated by a fifth function F5, namely, a property frequency signal (output signal Output).
As shown in FIG. 9. a bell filter BF is provided with 8 10-bit multipliers, 14 adders, 15 D-flip-flops and one coefficient unit ( 1/64). Fixed values (control signals) C0 to C7 are sent to the adders.
Japanese Laid Open Patent Application (JP-A-Heisei 1-152893) discloses a method of generating a color signal based on a next SECAM Standard by using a digital process. This method of generating the color signal based on the SECAM Standard by using the digital process has the following feature. That is, a Y signal, an R-Y signal and a B-Y signal are stored at a digital value, on which a pre-emphasis is performed, in a memory. The R-Y signal and the B-Y signal which are read out from the memory becomes conversion outputs on which a digital FM conversion and a sine wave conversion are performed. The Y signal read out from the memory is added to the conversion output. Then, the added outputs are converted from the digital to the analog. Accordingly, the analog color signal based on the SECAM Standard is taken out.
In the case of the video signal processing circuits of the PAL Standard and the NTSC Standard, the common usage between both the circuits is easy in that they use an AM signal. However, in the case of the video signal processing circuit of the SECAM Standard, the common usage between it and the video signal processing circuits of the PAL Standard and the NTSC Standard is difficult in that it uses an FM signal.
Also, if the bell filter is constituted by the FIR filter, the configuration of the circuit becomes large and complex. One of the reasons why the configuration of the circuit becomes large and complex is that it requires a digital filter of a high order (for example, 13 to 20 taps).